A conventional fixing device includes a fixing roller and a heat pipe (see JP 2004-77683 A). Therein, the fixing roller is heated by a heater. Recording paper is heated and pressurized for fixing by the fixing roller and the pressure roller.
The heat pipe is in contact with the pressure roller to keep surface temperature of the pressure roller uniform. Specifically, heat is transferred into the pressure roller by evaporation or condensation of operating fluid contained in the heat pipe.
However, the conventional fixing devices has a problem that the heat pipe may be excessively heated because the heat pipe receives heat from the fixing roller via the pressure roller. This increases pressure of the operating fluid in the heat pipe and thereby causes damage to the heat pipe. That is to say, the conventional fixing devices have not been equipped with any safety devices for preventing damage to the heat pipe.